Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/session/grpchijack/hijack.go

@@ -0,0 +1,15 @@
+package grpchijack
+
+import (
+	"net"
+
+	controlapi "github.com/moby/buildkit/api/services/control"
+	"google.golang.org/grpc/metadata"
+)
+
+// Hijack hijacks session to a connection.
+func Hijack(stream controlapi.Control_SessionServer) (net.Conn, <-chan struct{}, map[string][]string) {
+	md, _ := metadata.FromIncomingContext(stream.Context())
+	c, closeCh := streamToConn(stream)
+	return c, closeCh, md
+}